Disk drive with a dual-stage actuator and failure detection and recovery system for the secondary actuator

ABSTRACT

A dual-stage actuator disk drive has calibration tracks located in a nondata band, and uses a secondary-actuator failure detection and calibration test run by the servo control processor. The calibration tracks contain a pattern of pre-written magnetized test blocks. The test is run with the primary actuator biased against a crash stop, which enables the read head to access the calibration tracks. The servo control processor generates a test signal to the secondary actuator and receives a calibration signal from the read head as the read head detects the test blocks in the calibration tracks. The test comprises two measurements: a measurement of the secondary actuator static characteristics, and a measurement of the secondary actuator dynamic characteristics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to magnetic recording hard disk drives,and more particularly to a disk drive with a dual-stage actuator forpositioning the read/write heads.

2. Description of the Related Art

Magnetic recording hard disk drives with dual-stage actuators forpositioning the read/write heads on the disks have been proposed. Arotary voice-coil-motor (VCM) is typically the primary actuator, withthe secondary actuator attached to the VCM and the read/write headsattached to the secondary actuator. A servo control system receivesservo positioning information read by the read/write heads from the datatracks and generates control signals to the primary and secondaryactuators to maintain the heads on track and move them to the desiredtracks for reading and writing of data. As in conventional single-stageactuator disk drives, each read/write head is attached to the end of ahead carrier or air-bearing slider that rides on a cushion or bearing ofair above the rotating disk. The slider is attached to a relativelyflexible suspension that permits the slider to “pitch” and “roll” on theair bearing, with the suspension being attached to the end of the VCMactuator arm. The secondary actuator is typically a piezoelectric orelectrostatic milliactuator or microactuator located on the VCM actuatorarm for driving the suspension, or on the suspension between thesuspension and the slider for driving the slider, or on the slider fordriving just the read/write head.

The conventional servo control system for a typical dual-stage actuatordisk drive uses a controller designed to assure stability of the VCMwith adequate stability margins as if it were to operate without thesecondary actuator. Then the controller for the secondary actuator isdesigned to achieve the desired combined dual-stage bandwidth. Thesecondary actuator control loop and the combined dual-stage control loopare also designed to ensure adequate stability separately and jointlywith the other control loops. This type of servo control system issatisfactory for limited increases in the bandwidth above what isachievable with only the VCM.

In co-pending application Ser. No. 10/802,601 filed Mar. 16, 2004,titled “MAGNETIC RECORDING DISK DRIVE WITH DUAL-STAGE ACTUATOR ANDCONTROL SYSTEM WITH MULTIPLE CONTROLLERS”, and assigned to the sameassignee as this application, a dual-stage actuator disk drive isdescribed that operates with an improved servo control system that hastwo controllers. One controller is a dual-stage controller thatsimultaneously generates a primary actuator control signal and asecondary actuator control signal, and uses a degraded-stability primaryactuator controller design with relatively high low-frequency open-loopgain and a secondary actuator controller design that provides stabilityand high mid-frequency to high-frequency open-loop gain resulting inincreased bandwidth. The other controller is a single-stage controllerthat generates only a primary actuator control signal and uses a stableVCM-only controller design. If a potential failure of the secondaryactuator is detected, the servo control system selects the single-stagecontroller.

In dual-stage actuator disk drives with either the conventional servocontrol system or the control system of the co-pending application, afailure of the secondary actuator will result in reduced performance andmay lead to loss of data and/or failure of the disk drive.

What is needed is a dual-stage actuator disk drive that can detectactual failure of the secondary actuator and modify the controllerparameters if the failure is recoverable.

SUMMARY OF THE INVENTION

The invention is a dual-stage actuator disk drive that has calibrationtracks located outside the data region, such as in a nondata band beyondthe inside diameter of the data band, and uses a secondary-actuatorfailure detection and calibration test that is run by the servo controlprocessor. The calibration tracks contain a pattern of pre-writtenmagnetized test blocks that can be accessed and detected by the readhead when the primary actuator is biased against a crash stop. The servocontrol processor runs a secondary-actuator failure detection andcalibration test, which can be performed on a regular schedule or atselected times, such as at disk drive start-up. The test is run with theprimary actuator biased against a crash stop, which enables the readhead to access the calibration tracks but not the data tracks. The servocontrol processor generates a test signal to the secondary actuator andreceives a calibration signal from the read head as the read headdetects the test blocks in the calibration tracks.

The test comprises two measurements: a measurement of the secondaryactuator static characteristics, and a measurement of the secondaryactuator dynamic characteristics. In both measurements, the read headsignal amplitude from the test block pattern is used to measure thesecondary actuator movement across the calibration tracks.

The static characteristics measurement is a calculation of the secondaryactuator “stroke”, i.e., the amount of head radial movement across thecalibration tracks as a function of voltage input to the secondaryactuator, and a comparison of the calculated stroke to a predeterminedrange of acceptable stroke values. With the primary actuator biasedagainst the crash stop, a first test signal is applied to the secondaryactuator and the read head movement is measured using the read headsignal from the test block pattern. The secondary actuator strokeresulting from the test signal is calculated from several positionmeasurements. These measurements are averaged over several diskrotations. If the calculated stroke is outside the acceptable range,then this is an indicator that the secondary actuator has likely failed.

If the calculated stroke is within the acceptable range, then thedynamic characteristics measurement is made. This measurement isessentially a measurement of the plant frequency response of thesecondary actuator. With the primary actuator biased against the crashstop, a second test signal is applied to the secondary actuator and theread head movement is recorded. The second test signal for the dynamiccharacteristics measurement is a series of test signals, each asinusoidal signal at a constant frequency. The read head position ismeasured at each calibration sector during the application of theconstant frequency test signal, and the resulting gain and phase of theread head response are recorded along with the corresponding frequency.This is repeated for each frequency in the series of test signals. Thisenables the plant frequency response of the secondary actuator to bemeasured. If the response is significantly different from the expectedresponse, it is virtually assured that the secondary actuator hasfailed. If the measured frequency response shows minor changes, such asa minor increase or decrease in the gain, or a minor shift in thefrequency at which the maximum gain occurs, the controller parametersare adjusted or re-optimized. This re-optimization changes the values ofthe controller parameters in the memory accessible by the servo controlprocessor. The parameters that can be changed include parameters thataffect bandwidth or stability margins, notching of particularfrequencies such as the secondary actuator resonant frequency, activedamping of the secondary actuator resonance, or other performance,robustness, or stability metrics.

The invention is applicable to both the dual-stage actuator disk drivewith a servo control system having a conventional dual-stage controller,and the dual-stage actuator disk drive according to the previously-citedco-pending application that has both a dual-stage controller and aselectable single-stage controller.

In the dual-stage actuator disk drive according to the co-pendingapplication, a potential failure of the secondary actuator is detectedeither by providing a model of the dynamic response of the primary andsecondary actuators and comparing the modeled head-position with themeasured head-position, or by measuring the position of the secondaryactuator relative to the primary actuator with a relative-positionsensor and comparing the relative position to a modeled position of thesecondary actuator. Upon detection of a potential failure of thesecondary actuator, the single-stage controller is selected, the primaryactuator is moved to the crash stop, and the secondary-actuator failuredetection and calibration test is run. If the secondary actuator passesboth the static characteristics measurement test and the dynamiccharacteristics measurement test, then the dual-stage controller isre-selected. If the measured frequency response shows minor changes fromthe optimized frequency response, the controller parameters are adjustedor re-optimized prior to re-selection of the dual-stage controller.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a prior art single-stage actuator diskdrive.

FIG. 2 is an open-loop frequency response for a prior art disk drivewith only a single-stage actuator, typically a voice-coil-motor (VCM).

FIG. 3 is a view of a secondary actuator usable with the above-describedprior art disk drive.

FIG. 4 is a comparison of an open-loop frequency response for a priorart disk drive with a dual-stage actuator with a stable VCM controllerdesign and the open-loop frequency response of FIG. 2.

FIG. 5 is a block diagram of a disk drive with a dual-stage actuator andcontrol system according to this invention.

FIG. 6 is a comparison of the open-loop frequency response for thedual-stage controller with a degraded-stability VCM controller designaccording to this invention and the open-loop frequency response of FIG.2.

FIG. 7 shows the gain of the VCM open-loop frequency response for astable single-stage VCM controller, the gain improvement with theaddition of a secondary actuator, and the further gain improvement withthe use of an increased-gain degraded-stability VCM controller and asecondary actuator.

FIG. 8 is a schematic structure of the control system of this invention.

FIG. 9 is a flow chart for the operation of the disk drive of thisinvention.

FIG. 10 shows one example of a pattern of magnetized test blockspre-written in the calibration tracks that can be used in thesecondary-actuator failure-detection and calibration test for the diskdrive of this invention.

FIG. 11 shows the read head position measurement using the test blockpattern of FIG. 10.

FIG. 12 shows the test block pattern demodulation timing.

FIG. 13 shows an example of the read back signal amplitude distributionthrough the test blocks when the read head is located at the center ofone of the calibration tracks.

FIG. 14 is a flow chart of the details of the secondary actuator staticcharacteristics measurement in this invention.

FIG. 15 is a flow chart of the details of the secondary actuator dynamiccharacteristics measurement in this invention.

FIG. 16 shows a typical test signal at a fixed frequency and a typicalcalibration signal response for the measurement of the secondaryactuator dynamic characteristics.

FIG. 17 is an example of the plant frequency response for a typicalmicro-electro-mechanical system (MEMS) type microactuator locatedbetween the suspension and the slider.

DETAILED DESCRIPTION OF THE INVENTION

Prior Art

FIG. 1 is a block diagram of a conventional single-stage actuator diskdrive that uses servo positioning information located inangularly-spaced servo sectors for positioning the read/write heads. Thedisk drive, designated generally as 100, includes data recording disk104, a voice coil motor (VCM) 110 as the primary and only actuator, aninner-diameter (ID) crash stop 102 and an outer-diameter (OD) crash stop103 for the VCM 110, an actuator arm 106, a suspension 107, a headcarrier or air-bearing slider 108, a data recording transducer 109 (alsocalled a head, recording head or read/write head), read/writeelectronics 113, servo electronics 112, and servo control processor 115.

The recording head 109 may be an inductive read/write head or acombination of an inductive write head with a magnetoresistive read headand is located on the trailing end of slider 108. Slider 108 issupported on the actuator arm 106 by a suspension 107 that enables theslider to “pitch” and “roll” on an air-bearing generated by the rotatingdisk 104. Typically, there are multiple disks stacked on a hub that isrotated by a-disk motor, with a separate slider and recording headassociated with each surface of each disk.

Data recording disk 104 has a center of rotation 111 and is rotated indirection 130. The disk 104 has a magnetic recording layer withradially-spaced concentric data tracks, one of which is shown as track118. The disk drive in FIG. 1 is illustrated as a zone-bit-recording(ZBR) disk drive because the data tracks are grouped radially into anumber of annular data zones or bands, three of which are shown as bands151, 152 and 153, but the invention is fully applicable to a disk drivethat does not use ZBR, in which case the disk drive would have only asingle data band. The range of rotation of the VCM is between the IDcrash stop 102 and the OD crash stop 103, which define the ID and OD ofthe annular region where data can be written. Each data track has areference index 121 indicating the start of track. Within each band, thetracks are also circumferentially divided into a number of data sectors154 where user data is stored. If the disk drive has multiple heads,then the set of tracks which are at the same radius on all diskrecording layers is referred to as a “cylinder”.

Each data track also includes a plurality of circumferentially orangularly-spaced servo sectors. The servo sectors in each track arealigned circumferentially with the servo sectors in the other tracks sothat they extend across the tracks in a generally radial direction, asrepresented by radially-directed servo sections 120. The servopositioning information in each servo sector includes a servo timingmark (STM), a track identification (TID) code, and a portion of apattern of magnetized blocks or high-frequency bursts that are decodedto provide a head position error signal (PES).

The servo positioning information in the servo sectors is read by theread/write electronics 113 and signals are input to the servoelectronics 112. The servo electronics 112 provides digital signals toservo control processor 115. The servo control processor 115 provides anoutput 191 to VCM driver 192 that controls current to the VCM 110 toposition the head 109.

Within the servo electronics 112, the STM decoder 160 receives a clockeddata stream from the read/write electronics 113. Once an STM has beendetected, an STM found signal is generated. The STM found signal is usedto adjust timing circuit 170, which controls the operating sequence forthe remainder of the servo sector.

After detection of an STM, the track identification (TID) decoder 180receives timing information from timing circuit 170, reads the clockeddata stream, which is typically Gray-code encoded, and then passes thedecoded TID information to servo control processor 115. Subsequently,the PES decoder 190 (also called the servo demodulator) captures theposition information from read/write electronics 113 and passes aposition error signal (PES) to servo control processor 115.

The servo control processor 115 includes a microprocessor 117 that usesthe PES as input to a control algorithm to generate the control signal191 to VCM driver 192. The control algorithm recalls from memory a“controller” 116, which is a set of parameters based on the static anddynamic characteristics of the “plant” being controlled, i.e., the VCM110. The controller 116 is a “single-stage” controller because the diskdrive being described has only a primary actuator, i.e., VCM 110, andservo control processor 115 provides only a single output, i.e., signal191 to VCM driver 192. The control algorithm is essentially a matrixmultiplication algorithm, and the controller parameters are coefficientsused in the multiplication and stored in memory accessible by themicroprocessor 117.

The method of designing the controller 116 is well-known in the digitalservo control and disk drive servo control literature. The controllercan be designed in the frequency domain to achieve the desired open-loopinput-output frequency response of the VCM 110. The input-outputbehavior of a dynamic system at any frequency can generally be expressedby two parameters, the gain (G) and the phase (φ) representing theamount of attenuation/magnification and phase-shift, respectively. Thegain and phase of a dynamic system represent the frequency response ofthe system and can be generated by experiment. In disk drivesingle-stage servo control systems the controller 116 must be a stabledesign. FIG. 2 is an example of an open-loop frequency response 210 fora disk drive with only a single-stage actuator, i.e., VCM 110. Thesingle-stage controller 116 for this system assures stability. Forexample, at the gain zero-crossover, the phase margin is about 30degrees and at −180 degree phase the gain margin is about 5 dB. Also, ata natural resonance near 4 kHz, the phase is near zero such that thesystem is phase stable.

FIG. 3 shows one example of a secondary actuator usable with theabove-described disk drive. The secondary actuator is an electrostaticrotary microactuator 200 located between the slider 108 and thesuspension 107. This type of microactuator is described in detail inU.S. Pat. No. 5,959,808 and L. S. Fan et al., “ElectrostaticMicroactuator and Design Considerations for HDD Applications”, IEEETransactions on Magnetics, Vol. 35, No. 2, March 1999, pp. 1000-1005.Rotation of the microactuator 200 as represented by arrow 202 causesrotation of the slider 108 and thus movement of head 109 in thecross-track direction. The microactuator 200 maintains the head 109on-track, while the VCM 110 moves the slider 108 (and thus head 109)across the tracks. Other types of secondary actuators are alsowell-known, such as piezoelectric-based actuators. Also, the secondaryactuator may be located on the suspension or actuator arm to move asuspension or arm-section relative to the fixed actuator arm, as in U.S.Pat. No. 5,936,805, or between the slider and a slider-segment to movethe head relative to the slider, as in U.S. Pat. No. 6,611,399.

The conventional control system for a disk drive with a dual-stageactuator is similar to that described above except that there is asecond output from the servo control processor that is sent to thedriver for the secondary actuator, and the controller is a dual-stagecontroller. In the dual-stage control system, the VCM or primaryactuator is typically responsible for large-amplitude, low-frequencymotion and the microactuator or secondary actuator is typicallyresponsible for small-amplitude, high-frequency motion. The design of adual-stage controller for a hard disk drive dual-stage servo controlsystems is well-known, as described for example, in Y. Lou et al.,“Dual-Stage Servo With On-Slider PZT Microactuator for Hard DiskDrives”, IEEE Transactions on Magnetics, Vol. 38, No. 5, September 2002,pp. 2183-2185; and T. Semba et al., “Dual-stage servo controller for HDDusing MEMS microactuator”, IEEE Transactions on Magnetics, Vol. 35,September 1999, pp. 2271-2273. Generally, the design of a dual-stagecontroller starts with the VCM controller, typically with a design thatis very similar to a single-stage VCM controller, such as the design forthe VCM controller whose open-loop frequency response 210 is shown inFIG. 2. In particular, the stability of the VCM is assured with adequaterobustness or stability margins as if it were to operate without thesecondary actuator. Then the controller for the secondary actuator isdesigned to achieve the desired combined dual-stage bandwidth. Thesecondary actuator control loop and the combined dual-stage control loopare also designed to ensure adequate stability separately and jointlywith the other control loops. This process is satisfactory for limitedincreases in the bandwidth above what is achievable with only the VCM.An example of an open-loop frequency response for a dual-stage hard diskdrive with a conventional stable VCM controller design is shown asfrequency response 220 in FIG. 4 and compared with the frequencyresponse 210, which is also shown FIG. 2. The response 220 is similar toresponse 210 at low frequencies, but has higher gain in themid-frequency range, and a higher bandwidth. For disk drives in generaland for the frequency responses described herein, the low frequencyrange is generally meant to be below approximately 300 Hz, the midfrequency range is generally meant to be approximately 300 Hz to 2 kHz,and the high frequency range is generally meant to be aboveapproximately 2 kHz. However, the boundaries between what are consideredlow, mid, and high frequency ranges are more accurately linked to wherethe microactuator begins to dominate the overall frequency response andthe open loop bandwidth (0 dB crossover) achievable with a single-stageactuator. This is strongly related to the physical size of the diskdrive.

The Invention

In a dual-stage actuator disk drive as described above, failure of thesecondary actuator will result in reduced performance and may lead toloss of data and/or failure of the disk drive. Thus it is important tobe able to detect actual failure of the secondary actuator andre-optimize the controller parameters if the failure is recoverable.

This invention is a disk drive that has calibration tracks locatedoutside the data region, such as in a nondata band just beyond the ID orOD of the data band, and uses a secondary-actuator failure-detection andcalibration test run by the servo control processor. The calibrationtracks contain a pattern of pre-written magnetized test blocks that canbe accessed and detected by the read head when the VCM is biased againstthe crash stop. The servo control processor runs a secondary-actuatorfailure detection and calibration test, which can be performed on aregular schedule or at selected times, such as at disk drive start-up.The test is run with the VCM biased against a crash stop, preferably theID crash stop, which enables the read head to access the calibrationtracks but not the data tracks. The servo control processor generates atest signal to the secondary actuator and receives a calibration signalfrom the read head as the read head detects the test blocks in thecalibration tracks.

The test comprises two measurements: a measurement of the secondaryactuator static characteristics, and a measurement of the secondaryactuator dynamic characteristics. In both measurements, the read headsignal amplitude from the test block pattern is used to measure thesecondary actuator movement.

The static characteristics measurement is a calculation of the secondaryactuator “stroke”, i.e., the amount of head radial movement across thecalibration tracks as a function of voltage input to the secondaryactuator, and a comparison of the calculated stroke to a predeterminedrange of acceptable stroke values. With the VCM fixed against the IDcrash stop, a first test signal is applied to the secondary actuator andthe read head movement is measured using the read head signal from thetest block pattern. The secondary actuator stroke resulting from thetest signal is calculated from several position measurements. Thesemeasurements are averaged over several disk rotations. If the calculatedstroke is outside the acceptable range, then this is an indicator thatthe secondary actuator has likely failed.

The dynamic characteristics measurement is essentially a measurement ofthe plant frequency response of the secondary actuator. With the VCMfixed against the ID crash stop, a second test signal is applied to thesecondary actuator and the read head movement is recorded. The testsignal for the dynamic characteristics measurement is a series of testsignals, each a sinusoidal signal at a constant frequency. The read headposition is measured at each calibration sector during the applicationof the constant frequency test signal, and the resulting gain and phaseof the read head response are recorded along with the correspondingfrequency. This is repeated for each frequency in the series of testsignals. This enables the plant frequency response of the secondaryactuator to be measured. If the response is significantly different fromthe expected response, it is virtually assured that the secondaryactuator has failed. If the measured frequency response shows minorchanges, such as a minor increase or decrease in the gain, or a minorshift in the frequency at which the maximum gain occurs, the controllerparameters are adjusted or re-optimized. This re-optimization changesthe values of the controller parameters in the memory accessible by theservo control processor. The parameters that can be changed includeparameters that affect bandwidth or stability margins, notching ofparticular frequencies such as the secondary actuator resonantfrequency, active damping of the secondary actuator resonance, or otherperformance, robustness, or stability metrics.

While the invention is fully applicable to a dual-stage actuator diskdrive with a servo control system having a conventional dual-stagecontroller as described above, the invention will be described in detailbelow as implemented in the dual-stage actuator disk drive with theimproved servo control system of the previously-cited co-pendingapplication.

FIG. 5 is a block diagram of the control system of the present inventionfor a dual-stage hard disk drive. The head 109 reads the servo patternfrom the disk, the read/write electronics 113 processes the signal fromthe head 109, and the servo electronics 112 generates the PES from thesignals from read/write electronics 113, all as described in the priorart.

The servo control processor 400 receives the PES from servo electronics112, and provides a primary control signal 191 to VCM driver 192 and asecondary control signal 229 to microactuator driver 230. The servocontrol processor includes a microprocessor 117 and uses a dual-stagecontroller 410 to generate control signals 191, 229. The dual-stagecontroller 410 incorporates a degraded-stability VCM controller withrelatively high low-frequency open-loop gain, and a secondary actuatorcontroller providing stability to the dual-stage controller and highmid-to-high-frequency open-loop gain, resulting in increased bandwidth.However, if the microactuator 200 fails while the disk drive is underthe control of dual-stage controller 410, then VCM 110 will becomeunstable. If the microactuator 200 fails then the servo controlprocessor 400 switches to use of a single-stage stable controller 420and generates only a primary control signal 191 to VCM driver 192. Thesingle-stage controller 420 can be a VCM controller based on thefrequency response 210 (FIG. 2) or any VCM controller that is stablewithout the microactuator 200. This stable VCM controller 420 will mostlikely have decreased performance, but will prevent catastrophic failureof the disk drive that would result in loss of data.

In the preferred embodiment, the detection of potential failure ofmicroactuator 200 is by a microactuator relative-position sensor 240.The sensor 240 measures the displacement of microactuator 200 relativeto VCM 110 and provides a relative-position signal (RPS) to servocontrol processor 400. If the secondary actuator is an electrostaticmicroactuator, then sensor 240 can be a capacitance sensing circuit, asdescribed in M. T. White et al., “Use of the Relative Position Signalfor Microactuators in Hard Disk Drives”, Proceedings of the AmericanControl Conference, Denver, Colo., Jun. 4-6, 2003, pp. 2535-2540.

FIG. 6 is the open-loop frequency response 412 for the dual-stagecontroller 410 with a degraded-stability VCM controller design comparedwith the frequency response 210 for the single-stage stable controller420. As frequency response 412 shows, the low frequency gain may beincreased by relaxing the stability requirements of the VCM, but at theexpense of robustness. The resulting open-loop frequency response of thedual-stage system 412 has increased gain over a wider frequency comparedto the open-loop frequency response of the dual-stage system 220 shownin FIG. 4. This will result in better disturbance rejection andperformance. The frequency response of the dual-stage system 412 hasgain and phase margins that are comparable to the frequency response ofthe single-stage system 210 in FIG. 2. The phase margin near 2.8 kHz isabout 30 degrees and the gain margin near 3.8 kHz is about 5 dB.

FIG. 7 is the gain portion of three frequency responses and is agraphical explanation of the invention. Solid line 210 represents thegain of the VCM open-loop frequency response for a stable single-stageVCM controller. Dotted line 220 represents the improvement to response210 with the addition of the secondary actuator (microactuator 200) andis the typical shape for a conventional dual-stage controller. It hasincreased bandwidth and increased gain in the mid-frequency range. Thiswill result in improved disturbance rejection and faster response atthese frequencies. However, because the low-frequency gain is stilldetermined by the single-stage VCM controller, there is no improvementat low frequency. Cross-hatched line 412 represents the furtherimprovement with the use of an increased-gain degraded-stability VCMcontroller and the secondary actuator. This response also has increasedlow-frequency gain, and is comparable in shape to the response for theVCM-only design, but shifted higher in frequency. However, increasingthe low-frequency gain will also decrease the phase margin for the VCMcontroller, potentially to the point of instability of the VCM. Thesecondary actuator controller is then designed to make the combinedsystem stable, as well as increasing the mid-frequency to high-frequencygain. Using the secondary actuator to ensure the stability of thecombined system typically takes significantly less stroke than using thesecondary actuator to increase the low-frequency gain, and is thereforea more efficient use of the limited secondary actuator stroke to achievehigh bandwidth with adequate disturbance rejection.

With the dual-stage controller having the characteristics represented byline 412 in FIG. 7, failure of the secondary actuator results in anunstable system. This could lead to inoperability of the hard diskdrive, or even failure with loss of data. This is avoided by detecting apotential failure of the secondary actuator and switching to a stableVCM-only controller. The potential failure of the secondary actuator isdetected either by providing a model of the dynamic response of theprimary and secondary actuators and comparing the modeled head-positionwith the measured head-position, or by measuring the position of thesecondary actuator relative to the primary actuator with arelative-position sensor and comparing the relative position to amodeled position of the secondary actuator. In the present invention,after a potential failure of the secondary actuator has been detectedand switching to the VCM-only controller has occurred, thesecondary-actuator failure-detection and calibration test is run by theservo control processor.

A schematic structure of the control system of the present invention isshown in FIG. 8. C_(MACT) and P_(MACT) represent the microactuatorcontroller and plant, respectively, and C_(VCM) and P_(VCM) representthe VCM controller and plant, respectively. The controllers C_(MACT) andC_(VCM) together represent the dual-stage controller 410. Themicroprocessor 117 in servo control processor 400 (FIG. 5) runs thecontrol algorithm using the parameters of controllers C_(MACT) andC_(VCM) and generates control signals u_(MACT) and u_(VCM) (229 and 191,respectively, in FIG. 5). The control system includes a model 430 of themicroactuator plant and a model 440 of the VCM plant. These models maybe determined from frequency response measurements of the microactuatorand VCM, finite element models (FEM), or other well-known systemidentification techniques.

FIG. 8 shows two methods for determining potential failure of themicroactuator 200. In the preferred method the calculated microactuatorcontrol signal u_(MACT) is input to the microactuator model 430 and theestimated microactuator position y_(MACT(EST)) from the model iscompared to the RPS from sensor 240 at junction 450. In an alternative“PES-based” method the calculated microactuator control signal u_(MACT)is input to the microactuator model 430 and the calculated VCM controlsignal u_(VCM) is input to the VCM model 440. The modeled expected orestimated output y_(EST) is then compared with the measured outputy_(MEAS) at junction 460.

FIG. 9 is a flow chart for the operation of the disk drive of thepresent invention. The flow chart portion 500 describes the method fordetecting potential failure of the secondary actuator and switching tothe VCM-only stable controller if potential failure is detected, andflow chart portion 600 describes the method for detecting actual failureof the secondary actuator and possible re-optimization of the controllerparameters to enable switching back to the dual-stage controller havinga degraded-stability VCM controller.

Referring first to portion 500, the control system starts (block 501)and continues to operate using the dual-stage controller 410 with thedegraded-stability VCM controller (block 505). In block 510, theposition of the head is measured (y_(MEAS)) if the PES-based method isused, or the relative position of microactuator 200 is measured (RPS) ifthe relative position sensing method is used. In block 515 the expectedor estimated head position y_(EST) is calculated from the models 430,440 if the PES-based method is used, and the expected or estimatedrelative position (y_(MACT(EST))) is calculated from microactuator model430 if the relative position sensing method is used. The difference(DIFF) is then tested to see if it is within pre-determined bounds(block 520). If yes, the control continues (block 505).

If DIFF is outside the bounds, this indicates potential failure of themicroactuator 200. Once a potential failure of the secondary actuatorhas been detected the servo control processor 400 recalls the stable VCMcontroller 420 (FIG. 5) from memory (block 525). The processor sends acrash stop signal to the VCM 110 to move the head to the far innerdiameter of the disk. The VCM 110 is then biased so that it maintainsits position at the inner diameter with the ID crash stop 102 compressed(block 530). In this way, the position of the VCM is essentially fixed.Because the VCM is biased against the crash stop, the effects of othersources of error are virtually eliminated. This is a safe and accurateway to determine the motion of the microactuator in a condition that isvirtually isolated from the VCM. The operation then moves to the stepsdescribed in flow chart portion 600.

Referring to portion 600, the secondary-actuator static characteristicsmeasurement is performed (block 605). With the position of the VCM 110biased against the ID crash stop, a test signal is applied to themicroactuator 200 to calculate its stroke. At the check point (block610) if the stroke is two low or too high, as compared to predeterminedacceptable stroke values, then the secondary actuator has failed. Thesecondary actuator is disabled by selecting the VCM-only controller andan error is posted to the disk drive system (block 615). If thecalculated stroke meets the acceptable criteria, then thesecondary-actuator dynamic characteristics measurement is started (block620).

The secondary actuator dynamic characteristics measurement measures thesecondary actuator plant frequency response. At the check point (block625) if the measured response is significantly different from theexpected result, then the secondary actuator is disabled by selectingthe VCM-only controller and an error is posted to the disk drive system(block 630). If the measured response shows only minor changes, such asa minor gain increase or decrease, or a minor shift in the peakfrequency, then the secondary actuator can be recovered from potentialfailure (block 635). The controller parameters are then adjusted orre-optimized (block 640). The dual-stage controller with thedegraded-stability VCM controller and the new secondary actuatorcontroller parameters is re-selected (block 505) and operationcontinues.

FIG. 10 shows one example of a pattern of magnetized test blockspre-written or pre-recorded in the calibration tracks that can be usedin the secondary-actuator failure detection and calibration test. Thetest blocks are located in radially-directed calibration tracks in anondata band just beyond the ID of the data band. Each block can beeither a discrete magnetized region if the disk is a patterned disk withpatterned magnetic regions isolated from nonmagnetic regions, or a burstof high-frequency magnetic transitions if the disk is a conventionaldisk with a continuous magnetic recording layer. If the disk is aconventional disk, the burst pattern can be written during theservowriting process. The burst pattern is written across several trackswith interval spacing along the disk circumferential direction. Theblocks are generally a constant frequency pattern. In the example ofFIG. 10, the 16 bursts from calibration track 0 to calibration track 15are arranged in angularly-spaced calibration sectors. These calibrationsectors can be angularly spaced to coincide with the angular spacing ofthe servo sectors (see FIG. 1). Each sector is written with the samesector spacing interval, Ts, and the total number of sectors is n. Thecalibration track width Ws corresponds to one-half of the data trackwidth. Eight bursts (b0 to b7) form a first group and eight bursts (b8to b15) form a second group, with the first and second groups forming acalibration sector. This is called a 2×8 allocation pattern. Otherallocation patterns are possible, such as 4×4, 4×8, 1×16, etc. The indexsignal can be from the disk spindle motor commutation signal and isissued once per disk rotation. The burst pattern has a fixed timing(Ts0) from the index signal.

The secondary-actuator failure detection and calibration test comprisestwo measurements: a measurement of the secondary actuator staticcharacteristics and a measurement of the secondary actuator dynamiccharacteristics. The static characteristics measurement checks thesecondary actuator movement amount per some constant first test signal,and the dynamic characteristics measurement checks the plant frequencyresponse of the secondary actuator movement against a second test signalat one or more frequencies. In both measurements, the read head positionmeasurement is required to calculate the secondary actuator response tothe test signals.

FIG. 11 shows how the read head position is measured using the burstpatterns of FIG. 10. The secondary actuator input is set as the neutralbias. The read head center is moved to the center of calibration track 7(containing bursts b7) by adjusting the VCM current, and then the VCMcurrent is fixed during the remaining measurement process. The read backsignal amplitude of burst b7 now becomes the maximum. The burstamplitudes are measured during the timing window of the burst samplingsignal, which is set at a fixed timing from the index signal. The burstamplitudes are acquired at each sector. The read head position is thencalculated from the burst amplitude, as will be explained below. Thesame measurements are performed with a positive bias and a negative biason the secondary actuator input. Then the read head position at eachbias point can be calculated as a decimal multiple of the calibrationtrack width.

FIG. 12 shows the burst pattern demodulation timing. The disk driveread/write electronics demodulates the read back signal from the burstpattern. The read back signals at all 8 bursts are demodulated. Theburst pattern-sampling window is narrower than the burst pattern, sothat timing is tolerant to the burst pattern circumferential error dueto such effects as disk rotation jitter. FIG. 13 shows an example of theread back signal amplitude distribution through the bursts, when theread head is located at the center of burst b7. In this figure, b7 isthe largest amplitude and b6 and b0 (actually b8) are almost same andhalf of b7. In this example, the read head position can be calculated asPosition=7+½*(V8−V6)/(2*V7−V6−V8),where V6, V7, V8 are the amplitudes of the read back signal at burstsb6, b7, and b8, respectively. The read head position is expressed as adecimal multiple of the calibration track width.

In this example, the read back signal at burst b8 is sampled instead ofburst b0. This happens with the 2×8 allocation pattern when the readhead is located at the radial boundary of each 8-burst group. A simplealgorithm can be used to distinguish between bursts b0-b8, b1-b9,b2-b10, etc. For the dynamic characteristics measurement, the amplituderequired of the secondary actuator input is a very small value and thesecondary actuator movement is very small, typically less than one-halfthe calibration track width Ws, so that the initial read head positioncan be located at the center of burst b3 or b4.

1) Secondary Actuator Static Characteristics Measurement

FIG. 14 is a flow chart showing the details of the secondary actuatorstatic characteristics measurement (block 605 in FIG. 9). The secondaryactuator input is set as a neutral bias and the read head is moved tothe center of the calibration track with burst b7 by adjusting the VCMcurrent in very small steps (block 605 a). Next a constant input voltageis applied as a positive bias to the secondary actuator (block 605 b)and the read head position is measured (block 605 c) as described above.Next a constant input voltage is applied as a negative bias to thesecondary actuator (block 605 d) and the read head position is measured(block 605 e) as described above. The measurements (blocks 605 c and 605e) are done at each calibration sector and the average is calculated toget an accurate measurement. The secondary actuator stroke is thencalculated (block 605 f) as the measured movement of the read head forthe known positive and negative voltage inputs. The calculated stroke isthen compared with acceptable stroke values at check point 610 (FIG. 9).

2) Secondary Actuator Dynamic Characteristics Measurement

FIG. 15 is a flow chart showing the details of the secondary actuatordynamic characteristics measurement (block 620 in FIG. 9). Thismeasurement is performed after the static characteristics measurement.The secondary actuator input is set as a neutral bias and the read headis moved to the center of the calibration track with burst b7 byadjusting the VCM current in very small steps (block 620 a). Next thefirst frequency of the secondary actuator test signal is selected and asinusoidal input is made to the secondary actuator at the firstfrequency (block 620 b). Generally the amplitude of the sinusoidal inputsignal is held constant during the entire measurement. Then the readback signal amplitude is measured at each burst timing, and at eachsector for several disk rotations (block 620 c). FIG. 16 shows a typicaltest signal at a fixed frequency and a typical calibration signalresponse. (No units are listed on the vertical axis because FIG. 16 isintended to merely show the representative shapes for a test signal anda calibration signal response.) The circles on the calibration signalrepresent the measurements taken at the frequency at which thecalibration sectors pass the read head. The magnitude of the calibrationsignal response is shown as the peak values, and the phase can beroughly calculated as the difference between the zero crossings of thetest signal and the response. However, for a more accurate calculationof the gain and phase at the test signal frequency, a discrete Fouriertransform (DFT) is applied to the measured read head positions at theinput test signal frequency to calculate the gain and phase (block 620d). Then the gain and phase for that input test frequency are stored ina table (block 620 e). The next input frequency is selected for the testsignal (block 620 f) and the process is returned to block 620 b. Thesteps from blocks 620 b to 620 f are repeated until all desiredfrequencies are tested. The frequency range is generally limited to bebetween approximately the disk rotational frequency (1f) and one-halfthe sector sampling frequency. For the convenience of the DFTcalculation, multiples of the disk rotational frequency may be selected.After all desired frequencies have been tested, the table is searchedfor the maximum gain value and its corresponding frequency (block 620g).

After the maximum gain value has been obtained, the check point (block625 in FIG. 9) is performed according to the details shown in FIG. 15.The maximum gain value is checked to determine if it is within anacceptable range of predetermined values (block 625 a). For example, ifthe maximum gain is too low, e.g., not at least 20 db over the staticgain, this is an indication of actual failure of the microactuator(block 630 in FIG. 9). There are potential microactuator failure modesin which the motion is excessive, in which case the maximum gain valuemay be too high. If the maximum gain is not outside the range ofacceptable gain values, then the measured frequency at which the maximumgain occurs is compared to a predetermined acceptable range (block 625b). For example, if the maximum gain occurs at more than 1 kHz from theknown resonant frequency of the microactuator, this is an indication ofactual failure of the microactuator (block 630 in FIG. 9). However, ifthe frequency at which the maximum gain occurs is within acceptablelimits, then the controller parameters can be re-optimized (block 640 inFIG. 9). This re-optimization may take the form of a table lookout ofpre-calculated parameters or may be performed in real-time through theuse of an adaptive scheme. The controller may be adjusted to achievesuch features as desired bandwidth or stability margins, notching ofparticular frequencies such as the microactuator resonance, designingfor active damping of the microactuator resonance, or other performance,robustness, or stability metrics. However, if there is only a minimal orno substantial difference from the optimum values, then control can bereturned to block 505 (FIG. 9) without re-optimizing the controllerparameters.

FIG. 17 shows one example of the plant frequency response for a typicalmicro-electro-mechanical system (MEMS) type microactuator locatedbetween the suspension and the slider. The motion of this type ofmicroactuator is about 1 micrometer for a 30 V input at low frequency.So for the static characteristics test, a 0.5 micrometer movement wouldbe expected for a 15 V input. If the measured output was less than about0.2 micrometer or more than about 1 micrometer during the staticcharacteristics measurement, then this would likely indicate failure ofthe microactuator. In a server class disk drive, the spindle motorfrequency is between 167 Hz and 250 Hz, and the servo sector samplingfrequency is currently 50 kHz or above. So for the dynamiccharacteristics measurement, an acceptable appropriate frequency rangefor the series of input test signals would be from about 200 Hz to 10kHz. Thus as one example, the first sinusoidal test signal of FIG. 16could be at 167 Hz with subsequent test signals increased in 167 Hzincrements until the last test signal is reached at around 10 kHz. Thistype of microactuator also has a resonant frequency around 2 kHz, so ifthe maximum gain from the dynamic characteristics measurement is lessthan about 1 kHz or greater than about 3 kHz, then this would likelyindicated failure of the microactuator.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A disk drive comprising: a rotatable magnetic recording disk having aplurality of concentric data tracks in a data region and a plurality ofcalibration tracks in a nondata region, the data tracks containing servopositioning information and the calibration tracks containingradially-spaced magnetized test blocks; a recording head movable acrossthe disk, the head being capable of reading data and servo positioninginformation in the data tracks and detecting the test blocks in thecalibration tracks; a primary actuator for moving the head; a crash stopfor limiting movement of the primary actuator and located to enable thehead to detect the test blocks in the calibration tracks when theprimary actuator is at the crash stop; a secondary actuator connected tothe primary actuator, the head being connected to the secondaryactuator; and a servo control processor responsive to servo informationread by the head and comprising a dual-stage controller for generating aprimary actuator control signal and a secondary actuator control signal,the processor capable of sending a crash stop signal to the primaryactuator for moving the primary actuator to the crash stop and, when theprimary actuator is at the crash stop, generating a test signal to thesecondary actuator and receiving a calibration signal from the testblocks in response to the test signal.
 2. The disk drive of claim 1wherein the amount of movement of the head across the calibration tracksas a function of test signal value is the stroke of the secondaryactuator, and wherein the processor calculates the stroke.
 3. The diskdrive of claim 2 wherein the processor generates a failure signal if thecalculated stroke is outside a predetermined stroke range.
 4. The diskdrive of claim 1 wherein the test signal covers a range of frequenciesand the calibration signal is used to generate a frequency response. 5.The disk drive of claim 4 wherein the processor records the gains andcorresponding frequencies of the calibration signal frequency response.6. The disk drive of claim 5 wherein the processor generates a failuresignal if a recorded gain is outside a predetermined range of values. 7.The disk drive of claim 5 wherein the processor modifies the parametersof the controller from the recorded gains and corresponding frequencies.8. The disk drive of claim 4 wherein the processor also records thephases of the calibration signal frequency response.
 9. The disk driveof claim 1 wherein the dual-stage controller comprises adegraded-stability primary actuator controller having relatively highlow-frequency open-loop gain, and a selectable single-stage controllerfor generating only a primary actuator control signal.
 10. The diskdrive of claim 9 wherein the servo control processor includes a model ofthe dynamic response of the primary actuator and a model of the dynamicresponse of the secondary actuator, the models providing a modeledhead-position output when the control signals from the dual-stagecontroller are input to the models.
 11. The disk drive of claim 10wherein the single-stage controller is selected by the processor whenthe difference between the modeled head-position output and the measuredhead-position from the servo information detected by the head is greaterthan a predetermined value, and wherein the processor sends a crash stopsignal to the primary actuator when the single-stage controller isselected.
 12. The disk drive of claim 10 further comprising asecondary-actuator relative-position sensor coupled to the servo controlprocessor and wherein the secondary-actuator model provides a modeledoutput of the position of the secondary actuator relative to the primaryactuator when the secondary actuator control signal from the dual-stagecontroller is input to the secondary-actuator model, wherein thesingle-stage controller is selected by the processor when the differencebetween the modeled secondary-actuator relative position and themeasured relative secondary actuator position from the relative-positionsensor is greater than a predetermined value, and wherein the processorsends a crash stop signal to the primary actuator when the single-stagecontroller is selected.
 13. The disk drive of claim 1 wherein thesecondary actuator is an electrostatic microactuator.
 14. The disk driveof claim 1 wherein the secondary actuator is a piezoelectric actuator.15. A disk drive comprising: a rotatable magnetic recording disk havinga plurality of concentric data tracks in a data band and a plurality ofcalibration tracks in a nondata band, the data tracks containing servopositioning information and the calibration tracks containing a patternof radially-spaced magnetized test blocks; a recording head movableacross the disk, the head being capable of reading data and servopositioning information in the data tracks and detecting the test blocksin the calibration tracks; a voice-coil-motor (VCM) for moving the head;a crash stop for limiting movement of the VCM and located to enable thehead to detect the test blocks in the calibration tracks when the VCM isat the crash stop; a microactuator connected to the VCM, the head beingconnected to the microactuator; and a servo control processor responsiveto servo information read by the head and comprising (a) a dual-stagecontroller having a degraded-stability VCM controller with relativelyhigh low-frequency gain and a microactuator controller forsimultaneously generating a VCM control signal and a microactuatorcontrol signal, and (b) a selectable single-stage controller forgenerating only a VCM control signal, the processor being capable ofgenerating a test signal to the microactuator and receiving a headcalibration signal from the test blocks in response to the test signalwhen the single-stage controller is selected and the VCM is at the crashstop.
 16. The disk drive of claim 15 wherein the servo control processorincludes a model of the dynamic response of the VCM and a model of thedynamic response of the microactuator, the models providing a modeledhead-position output when the control signals from the dual-stagecontroller are input to the models.
 17. The disk drive of claim 16wherein the single-stage controller is selected by the processor whenthe difference between the modeled head-position output and the measuredhead position from the servo information read by the head is greaterthan a predetermined value.
 18. The disk drive of claim 17 wherein theamount of movement of the head across the calibration tracks as afunction of test signal value is the stroke of the secondary actuator,and wherein the processor calculates the stroke.
 19. The disk drive ofclaim 18 wherein the processor generates a failure signal if thecalculated stroke is outside a predetermined stroke range.
 20. The diskdrive of claim 17 wherein the test signal covers a range of frequenciesand the calibration signal is used to generate a frequency response. 21.The disk drive of claim 20 wherein the processor records the gains andcorresponding frequencies of the calibration signal frequency response.22. The disk drive of claim 21 wherein the processor generates a failuresignal if a recorded gain is outside a predetermined range of values.23. The disk drive of claim 21 wherein the processor modifies theparameters of the controller from the recorded gains and correspondingfrequencies.
 24. The disk drive of claim 21 wherein the processor alsorecords the phases of the calibration signal frequency response.
 25. Thedisk drive of claim 16 further comprising a microactuatorrelative-position sensor coupled to the servo control processor andwherein the microactuator model provides a modeled output of theposition of the microactuator relative to the VCM when the microactuatorcontrol signal from the dual-stage controller is input to themicroactuator model, and wherein the single-stage controller is selectedby the processor when the difference between the modeled microactuatorrelative position and the measured relative microactuator position fromthe relative-position sensor is greater than a predetermined value. 26.The disk drive of claim 25 wherein the amount of movement of the headacross the calibration tracks as a function of test signal value is thestroke of the secondary actuator, and wherein the processor calculatesthe stroke.
 27. The disk drive of claim 26 wherein the processorgenerates a failure signal if the calculated stroke is outside apredetermined stroke range.
 28. The disk drive of claim 25 wherein thetest signal covers a range of frequencies and the calibration signal isused to generate a frequency response.
 29. The disk drive of claim 28wherein the processor records the gains and corresponding frequencies ofthe calibration signal frequency response.
 30. The disk drive of claim29 wherein the processor generates a failure signal if a recorded gainis outside a predetermined range of values.
 31. The disk drive of claim29 wherein the processor modifies the parameters of the controller fromthe recorded gains and corresponding frequencies.
 32. The disk drive ofclaim 29 wherein the processor also records the phases of thecalibration signal frequency response.
 33. The disk drive of claim 15wherein the microactuator is an electrostatic microactuator.
 34. Thedisk drive of claim 15 wherein the microactuator is a piezoelectricmicroactuator.
 35. A method for operating a dual-stage actuator diskdrive, the disk drive including (a) a rotatable magnetic recording diskhaving a plurality of concentric data tracks in a data band and aplurality of calibration tracks in a nondata band, the data trackscontaining servo positioning information and the calibration trackscontaining a pattern of radially-spaced magnetized test blocks; (b) arecording head movable across the disk, the head being capable ofreading data and servo positioning information in the data tracks anddetecting the test blocks in the calibration tracks; (c) avoice-coil-motor (VCM) for moving the head; (d) a crash stop forlimiting movement of the VCM and located to enable the head to detectthe test blocks in the calibration tracks when the VCM is at the crashstop; (e) a microactuator connected to the VCM, the head being connectedto the microactuator; and (f) a servo control processor responsive toservo information read by the head and comprising (i) a dual-stagecontroller having a degraded-stability VCM controller with relativelyhigh low-frequency gain and a microactuator controller forsimultaneously generating a VCM control signal and a microactuatorcontrol signal, and (ii) a selectable single-stage controller forgenerating only a VCM control signal, the processor-implemented methodcomprising: detecting a potential failure of the microactuator;generating a crash stop signal to the VCM to move the VCM to the crashstop and selecting the single-stage controller if a potentialmicroactuator failure is detected; generating a first test signal to themicroactuator; receiving a head calibration signal from the test blocksin response to the first test signal; measuring the position of the headfrom the received calibration signal; calculating the stroke of themicroactuator from the first test signal and the measured head position;and posting a microactuator failure signal if the calculated stroke isoutside a predetermined stroke range.
 36. The method of claim 35 furthercomprising, if the calculated stroke is within the predetermined range,generating a series of second test signals to the microactuator, each ofthe second test signals in the series being at a frequency differentfrom the other second test signals in the series; receiving a series ofsecond calibration signals in response to the series of second testsignals; recording the gains and corresponding frequencies from each ofthe second calibration signals; and posting a microactuator failuresignal if the maximum recorded gain is outside a predetermined range.37. The method of claim 36 further comprising, if the maximum recordedgain is within a predetermined range, posting a microactuator failuresignal if the maximum recorded gain is at a recorded frequency outside apredetermined frequency range.
 38. The method of claim 36 furthercomprising, if the maximum recorded gain is at a recorded frequencywithin said predetermined frequency range, modifying the parameters ofthe controller from the recorded gains and corresponding frequencies.39. The method of claim 35 further comprising, if the calculated strokeis within the predetermined range, generating a series of second testsignals to the microactuator, each of the second test signals in theseries being at a frequency different from the other second test signalsin the series; receiving a series of second calibration signals inresponse to the series of second test signals; and recording the phasesand corresponding frequencies from each of the second calibrationsignals.
 40. The method of claim 35 wherein the servo control processoralso includes a model of the dynamic response of the VCM and a model ofthe dynamic response of the microactuator, and wherein theprocessor-implemented method further comprises: providing a modeledhead-position output when the control signals from the dual-stagecontroller are input to the models; and wherein detecting a potentialfailure of the microactuator comprises determining if the differencebetween the modeled head-position output and the measured head positionfrom the servo information read by the head is greater than apredetermined value.
 41. The method of claim 35 wherein the disk drivealso includes a microactuator relative-position sensor coupled to theservo control processor and the servo control processor also includes amodel of the dynamic response of the microactuator, and wherein theprocessor-implemented method further comprises: providing a modeledoutput of the position of the microactuator relative to the VCM when themicroactuator control signal from the dual-stage controller is input tothe microactuator model; and wherein detecting a potential failure ofthe microactuator comprises determining if the difference between themodeled microactuator relative position and the measured relativemicroactuator position from the relative-position sensor is greater thana predetermined value.